1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image on a medium, on which the toner image is to be fixed, in an image forming apparatus such as an electronic copying machine or a laser printer, and to a control method for the fixing device.
2. Description of the Related Art
In recent years, there is known a roller-fixation type fixing device that uses a heat roller with a small wall thickness in order to shorten a warm-up time for environmental protection.
In the case of using the heat roller with a small wall thickness, however, a temperature variation increases. In particular, immediately after warm-up, non-uniformity in temperature of the heat roller becomes conspicuous. While the small-wall-thickness heat roller is being rotated, an undershoot phenomenon occurs due to absorption of the heat of the heat roller by a press roller or transfer paper, which is in contact with the heat roller. This may lead to a defective fixed image.
There is a case where a temperature difference in the longitudinal direction of the heat roller increases depending on the width of paper that is fed in a successive copy mode. In particular, in the case of using small-width paper of, e.g. A4-R size, the temperature of that part of the heat roller, at which the paper does not pass, that is, the temperature of end portions of the heat roller, rises to an abnormal level. This may lead to damage to structural parts or a defective image, such as a high-temperature offset image, at a time of the next copying.
To solve this problem, there is a technique wherein a plurality of heater lamps with different light distribution characteristics are used, thereby dividing to-be-heated parts of the heat roller.
If plural heater lamps are used, however, there may be a case where the heater lamps cannot be turned on at the same time, depending on operational conditions, and a turn-on control, such as alternate turn-on, may be required. In the case of alternate turn-on, the turn-on time of each lamp needs to be increased to cope with a problem of flicker. Consequently, in the case of the heat roller using a plurality of heater lamps, the temperature variation increases and a defective image, such as a defective fixed image, may be formed when the temperature drops, for example, immediately after warm-up.
Besides, there is such a problem that the warm-up time increases, if the temperature at a time immediately after warm-up is set at a high level in consideration of the drop in temperature.